This invention relates to the recovery of refractory metal values from secondary resources such as scrap alloy.
A significant volume of alloy scrap of a type comprising one or more metals such as chromium, molybdenum, vanadium, and tungsten alloyed with one or more base metals such as iron, copper, nickel and cobalt is available in the United States. Examples of such materials include superalloys, corrosion and oxidation resistant alloys used in fabricating process equipment, steam and gas turbines and the like, spent desulfurization catalysts used in the petroleum industry, hardfacing grindings, tool steels, and related material.
While both the refractory metal values in such alloys as well as the nickel, cobalt and copper values represent a significant recycleable resource, attempts to recover these metals by an economically attractive process have not been very successful. Any recovery scheme involving remelting of the alloys necessarily requires a large and expensive energy input, especially in view of the high melting points of the refractory metals and the escalating costs of energy. Dissolution and selective extraction techniques which employ organic extractants can be designed for treating materials of a selected composition. However, the availability of scrap materials of more or less uniform composition is rather low and unpredictable. Consequently, a recovery process which is limited to the use of only one particular type of feed material cannot take advantage of the economics of scale and can often be placed in operation only on a noncontinuous basis. This results in a relatively poor utilization of capital equipment.
The inventions disclosed herein were developed with a view to designing an overall processing system for the recovery of both refractory metal values as well as nickel, cobalt, and copper from feed materials varying in the ratio of the various metals they contain, the identity of the metals alloyed together, and the state of oxidation, if any, of the metals. The logic of the approach was that a processing scheme capable of treating feeds of varying composition would largely overcome the problems arising from variation in the availability and cost of the various specific types of feed material and could be used on a continuous basis. The system was also sought to be designed so as to be able to handle conventional chromite and wolframite ores which could be mixed with the secondary feed materials as desired. Other broad objectives of the overall process disclosed herein are to reduce or eliminate the volume of effluents associated with the process and to eliminate environmentally dangerous effluents. Still another goal was to provide a secondary metal processing system which avoided any remelting step and which minimized reagent consumption.
The overall processing system involves a number of separate inventions which, in preferred embodiments, are employed in connection with each other. However, each of the separate inventions may be used individually in an appropriate situation, and it is not required that a single system embodying all the processes be used. In the overall processing scheme, feed materials are first calcined in the presence of an oxygen containing gas and an alkali metal carbonate, bicarbonate, or hydroxide. As a result of the calcination, chromium, vanadium, molybdenum and tungsten are converted respectively to chromates, vanadates, molybdates, and tungstates. The base metals are converted to water insoluble oxides. After the calcination, the mixture is water leached to produce a leach residue containing iron, nickel, cobalt, silicon, aluminum, and/or manganese and a leach liquor containing dissolved alkali metal chromate, tungstate, molybdate and vanadate. After preliminary purification, to remove soluble silicates, aluminates, and/or phosphates, the leach liquor is treated with alkaline earth metal ions, typically calcium ions, to selectively precipitate molybdenum, tungsten, and vanadium values as a mixed calcium cake. After separation of the molybdenum, tungsten, and vanadium values, the leach liquor, pregnant with chromium values, is reduced with carbon monoxide or low molecular weight oxygenated hydrocarbons. The reduction results in the precipitation of a hydrated chromium oxide product and in a spent liquor containing alkali metal salts of carbonate or bicarbonate which is dehydrated, the salts being recycled to the calcination step, and the water being recycled to leach.
Next, the calcium cake is treated with carbonated water or formic acid solution to selectively leach the vanadium values and some of the calcium. After separation of the vanadium containing liquor from the calcium molybdate and tungstate, the vanadium can be recovered by a variety of methods such as driving off some water and carbon dioxide content of the leach liquor, by adding a base such as calcium hydroxide to substantially quantitatively precipitate a product rich in V.sub.2 O.sub.5, or by treating the liquor with sulfuric acid to precipitate calcium sulphate and thereafter recovering a pure vanadium product by solvent extraction. The vanadium-barren calcium molybdate-calcium tungstate filter cake rejected from the vanadium stage is then repulped with water and the slurry is mixed with hydrogen peroxide and either sulfuric acid or a mixture of sodium bicarbonate and carbonated water. The presence of the hydrogen peroxide prevents the precipitation of molybdate and tungstate ions, and a calcium sulfate or calcium carbonate byproduct results which contains only trivial amounts of molybdate or tungstate. The molybdenum and tungsten values from the aqueous phase are then selectively extracted either by conventional techniques such as those disclosed in U.S. Pat. No. 3,969,478 or by heating the solution to decompose the peroxycomplexes and thus precipitate a hydrated WO.sub.3 solid, adding ammonia base to the solution to raise the pH thereof to between about 2 and 3, and heating the solution to precipitate a hydrated ammoniacal MoO.sub.3 product.
The process results in the production of valuable products of acceptable purity which are individually rich in chromium, vanadium, tungsten, and molybdenum. No aqueous effluents are produced, no energy intensive pyrometallurgical operation is involved, the reagents employed are all relatively inexpensive, and reagent consumption is minimized by recycle and other techniques.
Processes for calcining spent catalysts and other easily-calcined alloys are known in the art. However, the calcination and subsequent leaching of oxidation resistant superalloys is a novel approach to obtaining refractory metal values, and is disclosed in detail and claimed in copending application Ser. No. 140,428 filed Apr. 15, 1980. Processes for selectively recovering vanadium from a mixed alkaline earth metal solid filter cake containing molybdenum, tungsten, and vanadium values are disclosed in detail and claimed in copending application Ser. No. 140,569 filed Apr. 15, 1980. Processes for rejecting alkaline earth metal ion from a mixed alkaline earth metal molybdate and tungstate cake and subsequently recovering the molybdenum and tungsten values are disclosed in detail and claimed in copending application Ser. No. 140,436 filed Apr. 15, 1980. The overall Cr, Mo, V, and W recovery process is disclosed and claimed in copending U.S. application Ser. No. 140,437 filed Apr. 15, 1980. All of the foregoing applications were filed on even date herewith and their disclosures are incorporated herein by reference.
This application relates to the process of recovering hydrated chromium oxide from a solution containing Cr.sup.+6, and optionally, molybdenum, tungsten, and vanadium values.
It is known that solutions containing sodium chromate and/or dichromate can be obtained by calcining chromite ores. The production of sodium chromate and sodium dichromate from chromite ores has been reviewed by W. L. Faith et al in "Industrial Chemicals", John Wiley and Sons, Inc., New York, Third Edition, 1965, as well as by Kirk Othmer in "Encyclopedia of Chemical Technology" 2nd Ed. Volume 5, Interscience Publishers, John Wiley and Sons, Inc., New York. It is also known that, in acidic solutions, Cr.sup.+6 can be reduced to Cr.sup.+3 by a variety of methods using reagents such as sugars, aldehydes, and paper fibers, SO.sub.2, FeSO.sub.4, iron turnings, sodium sulfite and sodium thiosulfate. Catalytic reductions of Cr.sup.+6 to Cr.sup.+3 at low pH with hydrogen and carbon monoxide gas are disclosed in U.S. Pat. No. 4,033,867 and in chemical abstracts Nos. 65,16115 (1966). In alkaline media, reducing agents such as sulfur, sulfur dioxide and S.sup..dbd. ion have been utilized to precipitate Cr.sub.2 O.sub.3. Other patents of interest to this general technology include U.S. Pat. No. 2,544,687, British Pat. No. 748,610, U.S. Pat. 2,430,261, U.S. Pat. No. 4,066,734, U.S. Pat. No. 4,016,054, U.S. Pat. No. 4,024,474, and U.S. Pat. No. 4,054,517.